ScienceGrade 7ages 12–13
⚖️ Density
Density is mass divided by volume — comparing cubes of exactly the same size shows some materials, like steel, are far heavier than others, like wood, even at the same size, which is why a large wood block can still float.
Try this lesson freeNo card needed. The first lesson is free.
Download on the App StoreWhat your child learns
- To compare materials fairly, take cubes of exactly the same size.
- A cube of wood, one of plastic and one of steel are weighed, and the steel is far heavier.
- The size was identical, so the difference that remains is the material itself. That is density.
- Floating or sinking is decided against water: less mass than the same volume of water — it floats.
The mistake most kids make here
Bigger is always heavier, so it always sinks.
Try one question
Density is…
Density
- only the height in centimetres
- mass divided by volume
- the material's colour
Answer
mass divided by volume — Yes. Two measurements together, so it tells how much material sits in each unit of space.
How Maya teaches it
Your child tells Maya what they did in class. She teaches this topic at her desk, asks questions with tappable answers, and when the answer is wrong she explains why that answer was tempting instead of saying “try again”. She remembers what did not land and comes back to it.
Your child has it when they can: Say what density is, and why bigger is not always heavier.
How to help at home
"Bigger means heavier" holds true when comparing the same material — a bigger rock is heavier than a smaller rock. The fix is comparing different materials at the exact same size, which isolates what density really measures.
Tonight, if you have small same-sized blocks or toys made of different materials (wood, plastic, a metal object), let your child hold each and compare how heavy they feel for their size.
If your child insists a big wood block should sink because it's big, ask them to compare it fairly: imagine a same-size cube of steel next to it. The steel cube is far heavier — the wood isn't light because it's big, it's light because wood itself is a less dense material than steel.
A good sign they've got it: they can explain density as mass divided by volume, and say that whether something floats depends on comparing its density to water's, not on its size alone.
Questions parents ask
- Why does my child think bigger objects always sink?
- Because within the same material, bigger usually does mean heavier, it's a reasonable but incomplete rule that breaks down once you compare different materials at the same size.
- What is density, exactly?
- Density is an object's mass divided by its volume — it tells you how much material is packed into a given amount of space, which is why comparing same-sized cubes of different materials reveals real differences in density.
- Why can a big block of wood float while a small marble sinks?
- Wood has lower density than water, so it floats regardless of its size, while a marble (often glass or steel) has higher density than water, so it sinks even though it's small.
- How does Maya teach this differently from a worksheet?
- Maya asks what density is and offers "only the height in centimetres" as a wrong option, so a child has to connect mass and volume together rather than judge by size alone.
No card needed. The first lesson is free.
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Sources: Next Generation Science Standards